1. Field of the Invention
This invention relates to a driving force control system for a four-wheel drive vehicle, which controls the engagement forces of clutches to thereby control the drive forces distributed to auxiliary drive wheels.
2. Description of the Related Art
A driving force control system of this kind has been proposed by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 10-194005, for a four-wheel drive vehicle including left and right front wheels as main drive wheels, and left and right rear wheels as auxiliary drive wheels. The engine of the vehicle is connected to the left and right front wheels via an automatic transmission, a front differential, and left and right front drive shafts. Further, the front differential is connected to the left and right rear wheels via a transfer, a propeller shaft, a rear differential, and left and right drive shafts.
The driving force control system includes left and right electromagnetic clutches provided for the rear differential, an electronic control unit (xe2x80x9cECUxe2x80x9d) for control of these electromagnetic clutches, and a lock switch arranged on a dash board. The ECU controls the supply and cut-off of electric current to the solenoid coils of the respective clutches, whereby the clutches connect and disconnect the associated rear drive shafts to and from the propeller shaft. When these clutches are disengaged, the driving force of the engine is transmitted to the front wheels alone to thereby bring the vehicle into the front-wheel drive mode, whereas when the same are engaged, the driving force of the engine is also distributed to the rear wheels to bring the vehicle into the four-wheel drive mode. The engagement forces of the electromagnetic clutches are continuously changed according to the amounts of current supplied thereto, and the ECU controls the amounts of supplied current to vary the respective engagement forces of the left and right electromagnetic clutches whereby the driving forces distributed to the left and right rear drive wheels can be controlled independently of each other, as desired.
Further, when the driver operates the lock switch, the ECU executes a lock mode in which the amounts of current supplied to the clutches are made maximum to generate the maximum clutch engagement forces so as to maximize the driving forces distributed to the rear wheels. This makes it possible, for instance, to easily extricate the vehicle from a stuck condition on a snowy road.
However, the conventional driving force control system is configured such that the lock mode is unconditionally executed when the lock switch is operated. Therefore, if the driver operates the lock switch without the knowledge of the function of the lock switch or by mistake, the lock mode is unnecessarily executed. As a result, during this lock mode, the maximum amount of current continues to be supplied to the electromagnetic clutches to waste the electric power, and further fuel economy is degraded due to traveling of the vehicle in the four-wheel drive mode, which is fuel-consuming and, in this case, unnecessary. This can adversely affect the drivability as well, depending on the conditions of the vehicle. Further, the clutches are operated at their maximum engagement forces frequently and over extended time periods, so that the durability of the clutches is also degraded, which will offer an obstacle to the reduced size of electromagnetic clutches.
Further, in the conventional driving force control system, the lock mode is executed in a state where the clutch engagement forces are fixedly held at the maximum. On the other hand, after the vehicle gets unstuck from a stuck condition, the vehicle sometimes enters a traffic-congested traveling condition in which the running of the vehicle is controlled by engine brake or foot brake, with the accelerator pedal being scarcely stepped on by the user, and its lock switch being kept ON. In such a case, the engine output torque assumes a negative or small value, and hence it is almost unnecessary to distribute the driving force delivered to the front wheels to the rear wheels. Despite the fact, the vehicle continues to be in the lock mode in which the maximum clutch engagement forces are generated, and hence a large amount of electric power is wastefully consumed due to supply of the maximum amounts of current to the electromagnetic clutches.
It is a first object of the invention to provide a driving force control system for a four-wheel drive vehicle, which is capable of properly controlling the execution and cancellation of a lock mode in which the engagement forces of clutches for distributing a driving force to auxiliary drive wheels are made maximum, thereby reducing frequency and duration of the lock mode.
It is a second object of the invention to provide a driving force control system for a four-wheel drive vehicle, which is capable of properly controlling the engagement forces of clutches for distributing a driving force of the main drive wheels to auxiliary drive wheels, thereby causing the clutches to efficiently operate without waste of power.
To attain the first object, according to a first aspect of the invention, there is provided a driving force control system for a four-wheel drive vehicle including a pair of front wheels, a pair of rear wheels, an engine, a transmission for connecting one of the pair of front wheels and the pair of rear wheels to the engine, as main drive wheels, and clutches for connecting another of the pair of front wheels and the pair of rear wheels to the main drive wheels, as auxiliary drive wheels, the driving force control system controlling engagement forces of the clutches to thereby control distribution of a driving force of the main drive wheels to the auxiliary drive wheels.
The driving force control system according to the first aspect of the invention is characterized by comprising:
input means for being operated by a driver;
lock mode execution means for executing a lock mode in which the engagement forces of the clutches are set to predetermined maximum engagement forces, when the input means is operated;
low vehicle speed condition-determining means for determining whether or not the vehicle is in a predetermined low vehicle speed condition;
shift position-detecting means for detecting a shift position of the transmission; and
lock mode execution-permitting means for permitting the lock mode execution means to execute the lock mode, when the low vehicle speed condition-determining means determines that the vehicle is in the predetermined low vehicle speed condition, and at the same time the shift position-detecting means detects any of predetermined low-speed shift positions of the transmission.
According to this driving force control system, the lock mode execution means executes the lock mode in which the engagement forces of clutches for distributing a driving force to auxiliary drive wheels are made maximum, on condition that the input means is operated by the driver. Further, when the vehicle is determined to be in a predetermined low-vehicle speed condition, and at the same time the shift position of the transmission is detected to be any of predetermined low-speed shift positions, the lock mode execution-permitting means permits execution of the lock mode. The lock mode is executed by its nature mainly for the purpose of facilitating e.g. the extrication of the vehicle from a stuck condition, and hence the execution thereof is basically unnecessary when the vehicle is not in a low vehicle speed condition, including the stoppage of the same, or when the shift position of the transmission is in a medium or high speed position. Therefore, as described above, the lock mode is not executed only by operation of the input means, but is allowed to be executed on condition that the vehicle is in the predetermined low vehicle speed condition, and at the same time the shift position of the transmission is in any of the predetermined low-speed shift positions, whereby the lock mode can be executed depending on the conditions of the vehicle only when it is really required. This makes it possible to reduce the frequency and duration of execution of the lock mode.
Preferably, the driving force control system further includes lock mode-canceling means for canceling the lock mode after the lock mode execution means starts the lock mode, when the input means is operated.
More preferably, the lock mode-canceling means cancels the lock mode when the shift position-detecting means detects any shift position other than the predetermined low-speed shift positions of the transmission irrespective of whether the input means is operated or not.
According to this preferred embodiment, during execution of the lock mode, when the shift position of the transmission is shifted to any shift position other than the predetermined low-speed shift positions, the lock mode-canceling means cancels the lock mode in response to detection of the above-mentioned shift of the shift position by the shift position-detecting means. As mentioned above, the lock mode is executed mainly for the purpose of facilitating e.g. the extrication of the vehicle from a stuck condition, and hence basically unnecessary when the vehicle is not in a low vehicle speed condition. Therefore, when any shift position other than the predetermined low-speed shift positions is detected during the lock mode, by automatically canceling the lock mode without waiting for the driver to operate the input means, the lock mode can be canceled early, thereby reducing the duration of the lock mode.
More preferably, the lock mode-canceling means cancels the lock mode upon the lapse of a predetermined time period after the shift position-detecting means detects the any shift position other than the predetermined low-speed shift positions of the transmission.
According to this preferred embodiment, during the execution of the lock mode, even if the quick operation of the shift lever (i.e. operation of quickly reciprocatingly shifting the shift lever between a rear shift position for moving the vehicle rearward and a medium or high speed shift position, so as to make the vehicle get unstuck while causing forward and backward rocking motions of the vehicle) is carried out to make the vehicle unstuck, causing the shift lever to move across any shift position other than the predetermined low-speed shift positions, the lock mode is not cancelled but maintained, thereby enabling the vehicle to get unstuck, by the quick operation.
To attain the first object, according to a second aspect of the invention, there is provided a driving force control system for a four-wheel drive vehicle including a pair of front wheels, a pair of rear wheels, an engine, a transmission for connecting one of the pair of front wheels and the pair of rear wheels to the engine, as main drive wheels, and clutches for connecting another of the pair of front wheels and the pair of rear wheels to the main drive wheels, as auxiliary drive wheels, the driving force control system controlling engagement forces of the clutches to thereby control distribution of a driving force of the main drive wheels to the auxiliary drive wheels.
The driving force control system according to the second aspect of the invention is characterized by comprising:
input means for being operated by a driver;
lock mode execution means for executing a lock mode in which the engagement forces of the clutches are set to predetermined maximum engagement forces, when the input means is operated;
shift position-detecting means for detecting a shift position of the transmission; and
lock mode-canceling means for canceling the lock mode when the shift position-detecting means detects any shift position other than predetermined low-speed shift positions of the transmission, during execution of the lock mode.
According to this driving force control system, it is possible to obtain the same effects as obtained by the driving force control system according to the first aspect of invention which includes the lock mode-canceling means.
Preferably, the lock mode-canceling means cancels the lock mode upon the lapse of a predetermined time period after the shift position-detecting means detects the any shift position other than the predetermined low-speed shift positions of the transmission.
According to this preferred embodiment, it is possible to obtain the same effects as described as to the preferred embodiment of the first aspect of the invention.
To attain the second object, according to a third aspect of the invention, there is provided a driving force control system for a four-wheel drive vehicle including a pair of front wheels, a pair of rear wheels, an engine to which one of the pair of front wheels and the pair of rear wheels is connected as main drive wheels, and clutches for connecting another of the pair of front wheels and the pair of rear wheels to the main drive wheels, as auxiliary drive wheels, the driving force control system controlling engagement forces of the clutches to thereby control distribution of a driving force of the main drive wheels to the auxiliary drive wheels.
The driving force control system according to the third aspect of the invention is characterized by comprising:
driving force calculation means for calculating a driving force of the main drive wheels;
input means for being operated by a driver;
lock-mode engagement force-setting means for setting engagement forces of the clutches for a lock mode, including lockable clutch engagement forces which can lock the main drive wheels and the auxiliary drive wheels to each other, when the input means is operated; and
lock mode execution means for executing the lock mode based on the set engagement forces of the clutches for the lock mode.
According to this driving force control system, when the driver operates the input means, the lock-mode engagement force-setting means sets the engagement forces of the clutches (clutch engagement forces) for the lock mode. The lock mode execution means executes the lock mode based on the set clutch engagement forces for the lock mode. The clutch engagement forces for the lock mode includes lockable clutch engagement forces which can lock the main drive wheels and the rear drive wheels to each other, and hence the front and rear drive wheels can be locked to each other according to an intention of the driver. Further, the clutch engagement forces for the lock mode are set according to the driving force actually delivered from the engine to the main drive wheels, which makes it possible to properly distribute the driving force to the auxiliary drive wheels. This makes it possible to cause the clutches to operate efficiently without waste of power during the lock mode, and in the case of the clutches being electromagnetic clutches, electric power can be saved.
Preferably, the lock-mode engagement force-setting means sets the engagement forces of the clutches for the lock mode to smaller ones than the lockable clutch engagement forces, when the driving force of the main drive wheels calculated by the driving force calculation means is smaller than a predetermined value.
According to this preferred embodiment, the clutch engagement forces for the lock mode are limited to smaller ones than the lockable clutch engagement forces when the driving force of the main drive wheels calculated by the driving force calculation means is lower than a predetermined value. This makes it possible to limit the engagement forces of the clutches during the lock mode to smaller ones, when the necessity for distributing the driving force of the main drive wheels to the auxiliary drive wheels is small due to a small actual driving force of the main drive wheels. Further, since the clutch engagement forces are limited depending on the actual driving force delivered from the engine to the main drive wheels, it is possible to properly carry out the limitation of the clutch engagement forces. This makes it possible to cause the clutches to efficiently operate without waste of power, and in the case of the clutches being the electromagnetic clutches, electric power can be saved.
Preferably, the lock-mode engagement force-setting means sets the engagement forces of the clutches for the lock mode to smaller ones as the driving force of the main drive wheels is smaller.
According to this preferred embodiment, the clutch engagement forces can be finely limited according to the necessity of distribution of the driving force to the auxiliary drive wheels, which makes it possible to cause the clutches to operate more efficiently.
Preferably, the driving force control system includes vehicle speed detection means for detecting a traveling speed of the vehicle, and the lock-mode engagement force-setting means sets the engagement forces of the clutches for the lock mode to smaller ones as the traveling speed of the vehicle is larger.
More preferably, the lock-mode engagement force-setting means sets the engagement forces of the clutches for the lock mode to smaller ones dependent on the traveling speed of the vehicle, during deceleration of the vehicle in which the driving force of the main drive wheels assumes a negative value.
Preferably, the driving force calculation means calculates the driving force by using different calculating equations when the engine is in a normal condition and when the engine is in a faulty condition, respectively.
The above and other objects, features, and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.